How does the iPhone 7 Plus create a smooth zoom effect with two fixed lenses?
Asked 2/21/2017
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The iPhone 7 Plus uses two rear cameras with fixed focal lengths (roughly 28mm and 56mm equivalent). If neither lens has a variable optical zoom mechanism, how does the phone present a smooth, continuous zoom between those two views? Is it switching between cameras and digitally enlarging one image, or blending data from both cameras? How do differences in framing and distortion between the wide and tele cameras affect the result?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
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First of all: those objectives are fixed-focal.
One objective being 56mm means that it does not see anything around the frame what 28mm objective sees. You cannot interpolate between something known and something unknown, it is not interpolation.
There is only one convenient way in which this can be implemented:
- just one objective is used to record images at 28mm (or may be slightly past 28mm) or 56mm (those are 135-equivalents BTW)
- each image in-between has interpolated 28mm frame data (one may say "digitally zoomed") with 56mm frame data mixed into the central part to improve resolution of central part of the image in such a way that a resolution increase is not apparent at transition from wide objective data to tele objective data
- the resolution of output image being the same even though it does not mean anything at all: the outer part is interpolated to match one resolution throughout the range and the inner part is just partially mixed in to improve details
It is same interpolation which is used for digital zooming but it improves the central part of the image using "tele" objective.
Regarding the distortion: of course Apple would make it seamless too, it is a part of the trick. I hope that you do not mistake perspective as distortion, those are different concepts: perspective change results from shooting point change, geometric distortion is a property of optics. Perspective is exactly the same with any objective given that the shooting point is the same while distortion depends on the optics.
Usually the cropped central part of image made with some objective does not feature more distortions than image made with longer objective - the most distorted part of images is left outside of cropped area. I.e. second objective most probably does not improve distortion itself, it just improves detalisation.
Also geometry corrections are implemented in wide range of cameras for years already, so this means there are no real reasons for 28mm to exhibit more distortion at all. I do not know whether iPhone corrects distortion though.
Originally by user49477. Source · Licensed CC BY-SA 4.0
user49477
9y ago
0
Generated from our catalog & community — verify before relying on it.
The two iPhone 7 Plus cameras are fixed-focal-length lenses, not a true variable optical zoom. That means the phone cannot continuously change focal length optically like a zoom lens.
A smooth zoom effect is most plausibly created in software: the phone uses the wide camera at the short end, the tele camera at the long end, and for the in-between range it digitally crops/scales image data while likely blending or switching to the tele camera where its central detail can help. In other words, the “continuous” zoom is a user-interface effect, not continuous optical zoom.
Because the tele camera sees a narrower field of view, it does not capture everything the wide camera sees, so this is not simple interpolation between identical frames. Distortion and perspective can also differ between the two lenses, so the phone must correct and align the images in software to make transitions less obvious.
So: no moving zoom optics, no true continuous optical zoom—mainly camera switching plus digital processing.
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